Matic swapped Stamford Bridge for Old Trafford in July and has impressed in a red shirt.
“It’s the worst [decision] I’ve ever seen, if I can say,” Wilkins said on Sky Sports.
“I cannot see that for the life of me.
“You’ve got a guy here who was so paramount to Chelsea along with Kante in the middle of the pitch, winning the championship with Chelsea the year before and all of a sudden you’ve sold him to one of or the biggest rival we have.
“I say we because I am a Chelsea supporter so I’m speaking from the heart when I speak.
“I just could not see that for the life of me, why would we let Nemanja go there?”